Fossil fuel is a general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years. The utilization of fossil fuels has enabled large-scale industrial development and largely supplanted water driven mills, as well as the combustion of wood or peat for heat. Fossil fuels are a finite, non-renewable resource. The burning of fossil fuels by humans is a source of emissions of carbon dioxide. In addition, other air pollutants, such as nitrogen oxides, sulfur dioxide, volatile organic compounds (VOCs) and heavy metals are produced. The concentrations of several greenhouse gases, such as carbon dioxide, have increased over time. According to the global warming hypothesis, an increase in greenhouse gas emissions from industry and agriculture has played a role in global warming.
Increased demand for energy by the global economy has also placed increasing pressure on the cost of hydrocarbons. Aside from energy, many industries, including plastics and chemical manufacturers, rely heavily on the availability of hydrocarbons as a feedstock for their manufacturing processes. Cost-effective alternatives to current sources of supply could help mitigate the upward pressure on energy and these raw material costs.
One approach for the cost-effective alternatives utilizes free fatty acid production by microorganisms. Culturing the microorganisms, however, is difficult as a high level of free fatty acid in a culture environment is toxic to many microorganisms. For example, a culture of fatty-acid producing microorganisms may accumulate a toxic level of free fatty acids over a period of hours or days. In addition, the toxicity of the free fatty acid in the culture environment may cause a selective pressure to reduce the expression levels of genes associated with free fatty acid production by the microorganisms.